Utilities bury their pipes and/or cables (hereinafter “conveyances”) underground for many reasons, including protection from the elements, aesthetics, and the like. Such conveyances may be buried in the sea floor, deep beneath the surface of the water. To enable the buried conveyance to be located in the future, the utility typically records the location on a map. Contractors who are seeking to excavate in the vicinity of a buried conveyance must know the precise location of the conveyance to avoid damaging the conveyance or any peripheral components during the excavation. Since most maps do not provide enough detail to accurately locate a buried utility conveyance, several approaches have been proposed using radio frequency (RF) devices.
In a known method for locating a buried utility conveyance, an alternating current is impressed on a metallic portion of the conveyance or onto a separate associated conductor. A technician seeking to locate the conveyance utilizes a receiver to detect the signal radiated by the conveyance in response to the impressed signal. An improvement on this practice is disclosed in U.S. Pat. No. 5,644,237, assigned to the present assignee (AT&T Corp.) and incorporated by reference herein. This device utilizes a locating signal (tone) that comprises an RF signal at a frequency specific to the utility responsible for the conveyance of interest. The confirmation signal (tone) comprises a near DC signal that is pulsed in a particular manner unique to the conveyance of interest. To generally locate a particular conveyance, a technician utilizes a first detector for detecting the locating signal. After the general location is determined, the conveyance is precisely located with a second detector by detecting the strength of the confirmation signal.
U.S. Pat. No. 5,798,644, assigned to the present assignee (AT&T Corp.) and incorporated by reference herein, discloses a method and apparatus for locating buried conveyances that are spaced in close proximity to one another. Specifically, the '644 patent discloses a device which impresses a first (conveyance locating) signal, and a second (conveyance-confirmation) signal at a low frequency on the conveyance. The first signal causes the conveyance to radiate a first magnetic field. The conveyance-confirmation signal is pulsed in a particular pattern unique to the conveyance, causing it to radiate a second magnetic field unique to the conveyance. A first sensor senses the first magnetic field to locate the region of greatest strength for the first magnetic field, thereby identifying the region where the conveyance is generally disposed. An array of second sensors (magnetometers) are utilized to find the second magnetic field within the region where the first magnetic field is of greatest strength. A processor determines the location of the conveyance by vectoring signals from the second sensors caused by the second magnetic field.